Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1950 New horizons in string theory bubble babble in search of darkness SUVENDU GIRI ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-513-0982-8 UPPSALA urn:nbn:se:uu:diva-416673 2020 Dissertation presented at Uppsala University to be publicly examined in Häggsalen, 10132, Ångström, Lägerhyddsvägen 1, Uppsala, Thursday, 17 September 2020 at 13:00 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Professor Bert Vercnocke (KU Leuven, Institute for Theoretical Physics). Abstract Giri, S. 2020. New horizons in string theory. bubble babble in search of darkness. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1950. 120 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0982-8. It was discovered nearly two decades ago that we live in an accelerating universe that is dominated by dark energy. Understanding the origin of such an energy has turned out to be a very difficult open question in physics, and calls on the need for a fundamental theory like string theory. However, despite decades-long effort, string theory has proven incredibly resilient to a satisfactory construction of dark energy within its framework. In the first part of this thesis and the included papers, we examine this problem and propose two possible solutions. The first is a construction within the framework of M-theory, the eleven dimensional cousin of string theory. Using only well-understood geometric ingredients and higher-derivative corrections to eleven dimensional supergravity, we present a new class of four dimensional vacua that contain dark energy. In the process, we also construct a new class of non- supersymmetric Minkowski vacua that were previously not known. Our second idea is a novel proposal that our universe could be embedded on the surface of an enormous spherical bubble that is expanding in a five dimensional anti de Sitter spacetime. The bubble is made of branes in string theory and its expansion is driven by the difference in the cosmological constants across it. We argued that such a construction arises naturally in string theory, and showed how four dimensional gravity arises in such a universe. We further showed that four dimensional matter and radiation arise from quantities that are innately five dimensional. Another challenging problem in physics concerns the nature of black holes – the presence of an event horizon in particular. This poses a paradox between well understood physical principles, and requires a fundamental theory for its resolution. Towards this goal, we constructed a novel class of horizonless objects that mimics black holes, and proposed these objects as an alternative end point of gravitational collapse. Subsequently, we constructed slowly rotating versions of these "black shells" and proposed an observational signature that could distinguish them from black holes in cosmological experiments. This is discussed in the second part of the thesis and in the included papers. Keywords: String theory, black holes, dark energy, de Sitter, cosmological constant, M-theory, braneworld, anti de Sitter Suvendu Giri, Department of Physics and Astronomy, Theoretical Physics, Box 516, Uppsala University, SE-751 20 Uppsala, Sweden. © Suvendu Giri 2020 ISSN 1651-6214 ISBN 978-91-513-0982-8 urn:nbn:se:uu:diva-416673 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-416673) “What makes the desert beautiful,” said the little prince, “is that somewhere it hides a well...” — Antoine de Saint-Exupéry, The Little Prince Cover image credit: © User: Colin / Wikimedia Commons / CC BY-SA 3.0 List of papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I Johan Blåbäck, Ulf Danielsson, Giuseppe Dibitetto, and Suvendu Giri. Constructing stable de Sitter in M-theory from higher curvature corrections, JHEP 09 (2019) 042, arXiv:1902.04053. II Souvik Banerjee, Ulf Danielsson, Giuseppe Dibitetto, Suvendu Giri, and Marjorie Schillo. Emergent de Sitter Cosmology from Decaying Anti–de Sitter Space, Phys. Rev. Lett. 121 (2018) 26, 261301, arXiv:1807.01570. III Souvik Banerjee, Ulf Danielsson, Giuseppe Dibitetto, Suvendu Giri, and Marjorie Schillo. de Sitter Cosmology on an expanding bubble, JHEP 10 (2019) 164, arXiv:1907.04268. IV Souvik Banerjee, Ulf Danielsson, and Suvendu Giri. Dark bubbles: decorating the wall, JHEP 04 (2020) 085, arXiv:2001.07433. V Ulf Danielsson, Giuseppe Dibitetto, and Suvendu Giri. Black holes as bubbles of AdS, JHEP 10 (2017) 171, arXiv:1705.10172. VI Ulf Danielsson and Suvendu Giri. Observational signatures from horizonless black shells imitating rotating black holes, JHEP 07 (2018) 070, arXiv:1712.00511. Reprints were made with permission from the publishers. Contents 1 Introduction ................................................................................................. 9 Part I: Dark energy in string theory .............................................................. 13 2 Dark energy and the cosmological constant ......................................... 15 2.1 What is dark energy? ..................................................................... 15 2.2 Measuring dark energy ................................................................. 17 2.3 Dark energy and the cosmological constant .............................. 19 2.4 A fine-tuning problem .................................................................. 23 2.5 The path forward ........................................................................... 24 3 de Sitter in string theory ........................................................................... 25 3.1 Vacuum energy in supergravity ................................................... 26 3.2 Moduli stabilization ....................................................................... 27 3.3 The KKLT construction ................................................................ 29 3.4 Discussion ....................................................................................... 36 3.5 Other dS constructions .................................................................. 37 3.6 What about the fine tuning problem? ........................................ 39 4 de Sitter in M-theory ................................................................................. 42 4.1 What is M-theory? ......................................................................... 42 4.2 G2 holonomy and G2 structure .................................................... 44 4.3 dS in M-theory ................................................................................ 46 4.4 dS from higher-derivative corrections ........................................ 47 4.5 Summary ......................................................................................... 50 5 The landscape vs the swampland ........................................................... 51 5.1 Weak gravity conjecture ................................................................ 51 5.2 Generalized WGC .......................................................................... 53 5.3 Non-supersymmetric AdS conjecture ......................................... 54 5.4 de Sitter conjectures ...................................................................... 55 5.5 Possible counter examples ............................................................ 56 5.6 What does this mean for string theory? ..................................... 58 6 Revisiting an old idea: Braneworlds ....................................................... 59 6.1 Braneworlds .................................................................................... 59 6.2 The model ....................................................................................... 60 6.3 Four dimensional Planck scale .................................................... 62 6.4 Gravity on the braneworld ........................................................... 62 6.5 (A)dS braneworlds ......................................................................... 66 7 Introducing a new idea: Shellworlds ..................................................... 68 7.1 False vacuum decay ....................................................................... 68 7.2 Empty dS universe ......................................................................... 72 7.3 Populating the universe with matter and radiation ................. 75 7.4 Brane bending ................................................................................ 76 7.5 Gravity on the shellworld ............................................................. 77 7.6 Swampland conjectures ................................................................ 79 7.7 Support from string theory ........................................................... 80 7.8 What next? ...................................................................................... 81 Part II: Black holes .......................................................................................... 83 8 Black shells ................................................................................................. 85 8.1 Black hole thermodynamics ........................................................ 85 8.2 Information paradox ..................................................................... 88 8.3 Resolution of the information paradox ...................................... 89 8.4 Spinning black objects